The methods of generating electromagnetic radiation in the THz domain may be subdivided into three broad categories: radiation of fast moving electrons, microelectronic devices and optical techniques.
The first category includes synchrotron radiation, including free electron lasers and backward-wave tubes. The most versatile source of THz radiation is undoubtedly a free electron laser (FEL). Such a laser makes relativistic electrons oscillate between the poles of carefully positioned magnets, which generate tunable, coherent and high power radiation. The radiation wavelength is determined by the magnet period and electron energy. FELs require enormous resources, space and a dedicated staff to run the facilities. The cost, weight and size of a FEL are prohibitive.
The source of the THz emissions from backward-wave tubes is the spiraling motion of bunched electrons in strong magnetic fields. Backward wave tubes generate narrow-band, tunable radiation with average powers ˜1 mW. Operation of such a source requires a strong magnetic field and a very high voltage power supply, making it cumbersome and inefficient.
The second category, namely, electronic techniques of generating THz radiation are very attractive because of their promise of device integration and miniaturization; however, the speed limitations of the current microelectronics prohibit a practical transistor circuit from oscillating in the desired frequency range, i.e., above 0.557 THz. So far, the most practical and therefore most widely used way of reaching THz range is by multiplying the frequency of microwave oscillators using nonlinear devices such as diodes. Chains of lower order multipliers (lower than fourth order) are considered efficient THz sources. Though reported output powers are improving continuously, they are still quite low, ca. 0.1 mW at 1 THz, and drop quickly at higher frequencies.
The third category of optical approaches include THz lasers, various photoconductive emitters and nonlinear optical techniques. IR-pumped gas lasers are commercially available from Coherent Inc., DEOS, Edinberg Instruments, and MPE Technologies. Grating-tuned CO2 lasers pump low-pressure, flowing-gas cavities to produce THz waves. Though the output powers of THz gas lasers reach 100 mW, they are very bulky and power-inefficient.
Another type of THz laser is the Quantum Cascade Laser (QCL), a semiconductor device that operates on inter-sub-band transitions. When an electric current flows through the multiple layer structure of a QCL, electrons cascade down the energy staircase of inter-sub-bands and emit a photon at each step. These lasers emit narrow-band tunable radiation with ˜1 mW of average power. However, the operation of QCLs requires cryogenic equipment to keep the device at liquid He temperatures. Room temperature QCLs are deemed unlikely.
Photoconductive techniques rely either on mixing optical fields on low-temperature GaAs or on generating photocurrents in switches connected to various antenna structures. The efficiency of such devices at high frequencies is limited by low optical damage thresholds of inherently absorbing semiconductor materials and by the fundamental limitations arising from carrier lifetime/carrier collection efficiencies. Typically, photoconductive devices operate at sub-mW power levels and with conversion efficiencies of ˜10−4. It might be possible to combine a very large number of photoconductive emitters and thereby reach desirable power levels. Such a brute-force approach, however, requires phase-matching of individual emitters, which further complicates the design and, therefore, increases the technical complexity while reducing the overall efficiency and robustness of the resulting system.
Nonlinear optical techniques include short pulse rectification, difference frequency generation in phase-matched or quasi-phase-matched (QPM) materials and optical parametric oscillation (OPO). Pulse rectification occurs when a pulse propagates in a nonlinear crystal with a nonzero second-order nonlinear susceptibility, thus producing a traveling dipole that generates a single-cycle electric field resembling the first derivative of the optical pulse. The efficiency of the process is greatly increased when the generated THz field is phase-matched to the group velocity of the optical pulse. This is achieved either in a birefringent crystal, e.g., ZnTe, or by tilting the energy front of the optical pulse.
Coherent and tunable THz radiation can be produced by mixing two wavelength-offset optical frequencies in a phase-matched nonlinear material. This technique produced the highest conversion efficiency from the near IR to THz, reaching 0.024% at 1.53 THz according to W. Shi, Y. J. Ding and N. Fernelius “Improvement on Tuning Ranges and Output Powers by Means of Phase-Matches Difference-Frequency Generation in Zinc Germanium Phosphide” Appl. Phys. Lett., v. 83, 848 (2004). A modification of this technique is THz generation in a parametric oscillator. All these approaches rely on birefringence of nonlinear materials to achieve phase-matching between the near IR pump and seed waves and the THz radiation. The phase matching requirements limit the choice of available materials down to just a few, most notable examples of those being GaSe, LiNbO3, and ZnGeP2. Recently, a THz waveguiding technique has been proposed that tunes the phase velocity of the THz guiding wave to that of the optical beam without relying on material birefringence. This approach broadens the choice of material for THz generation. Yet another techniques that achieves relatively efficient conversion from near IR to THz is quasi phase matching in periodically-poled lithium niobate or in diffusion bonded GaAs and GaP.
It has been already mentioned that short pulse excitation techniques produce broadband THz waveforms that resemble the first derivative of the optical pulse shape, whereas CW and long pulse (quasi CW) excitation produces nearly monochromatic radiation. There is, however, a short-pulse excitation technique that generates long slices of THz sine waves. It is optical pulse rectification in Periodically Poled Lithium Niobates (PPLNs). When a short optical pulse propagates through a crystal with inverted ferroelectric domains, it creates electric fields directed in the opposite directions. Since THz and optical propagation speeds differ significantly, these fields add up into a THz waveform whose shape mimics the domain structure of the crystal. This technique not only allows generation of arbitrarily-shaped waveform, but also allows one to map the domain structure of the crystal with sub-micron resolution.
It is also important to emphasize that a nonlinear dipole propagating through a PPLN structure emits THz radiation in all directions, and that the observed THz frequency depends on the observation angle. Therefore, one may design the domain structure of PPLN for generating THz radiation with a particular frequency in the direction normal to the direction of the optical pulse. Since the cross section of the PPLN does not have to be much larger than the size of the optical beam, the side-emitted THz traverses only a short length of the crystal before exiting into the free space, thus reducing significantly the absorption due to the dielectric losses in the material.
TABLE ICharacteristics of nonlinear optical THzConversionTHzEfficiencyExcitation SchemeTHz SpectrumDirectionalityLimitLong-Pulse (quasi-NarrowbandHighlyManely-Rowe,cw) DFG and OPOdirectional~1%Short pulseBroadbandMay be100%rectificationdirectional ifphase-matchedShort pulse in PPLNNarrowband in aNon-directional100%particulardirection
Table I lists the characteristics of various nonlinear optical techniques of generating THz radiation. Long pulse (quasi CW), difference frequency generation (DFG) and OPO produce highly directional, narrowband radiation, and its demonstrated conversion efficiency approaches 5% quantum efficiency. This scheme, however has one fatal flaw—it cannot overcome the Manely-Rowe limit. At 100% quantum conversion efficiency, when all the high frequency photons of the pump are converted to signal and THz photons, the power conversion efficiency is under 1% because of the large difference in the THz and opticalnear IR frequencies. The Manely-Rowe limitation may be overridden with a cascading scheme, where the longer wavelength photons of the pump begin generating THz before the shorter wavelength photons are completely consumed by in the process of difference frequency generation (DFG). Such cascading schemes have been realized in Raman scattering of visible and near IR light at very high intensities that exceed significantly the intensity required for 100% conversion to the first Stokes wavelength. It is believed that no such schemes have been ever developed and demonstrated for THz generation.
Short optical pulses may in principle provide higher conversion efficiency, since the latter is proportional to the peak power of the pulse in the nonlinear schemes. It is true that the peak intensity of an optical pulse can only be increased up to the damage threshold of the nonlinear material. However, short optical pulses are advantageous over long ones from this perspective, since the damage threshold of transparent materials (in Watt/cm2) falls off for shorter pulses, until it scales with energy density for durations of 1 ps and less. Moreover, short pulse excitation schemes in principle are not limited by the Manely-Rowe restriction, because such pulses have sufficient spectral width for DFG within their bandwidth. As THz radiation consumes more and more power on the blue side of the pulse spectrum, the spectrum rolls to the red, which is similar to Raman red shifts observed for short optical pulses in fibers. This process may continue until most of the optical energy is converted to a THz wave, subject to significant degradation of the pulse shape due to dispersion.
Short pulse THz generating schemes, however, have other significant flaws. They produce broadband radiation, which is very hard to collect with high efficiency and utilize at the target.
What is needed is a method and apparatus for THz electromagnetic radiation generation that offers the best features of the above-described schemes, while not suffering from their drawbacks. The embodiments of the present disclosure answer these and other needs.